Various kinds of image displays, e.g., a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescence display (ELD), a cathode ray tube (CRT), a fluorescent character display tube and an electric field emission display, have been developed and apparatuses incorporated these image displays have been put to practical use in recent years.
These image displays are confronted with various problems, e.g., a problem that color correction is necessary since color purity and color separation of display elements are insufficient, a problem that contrast is reduced since backgrounds are reflected on displays, and a problem of leaking out of infrared rays and electromagnetic waves attributable to display elements. For coping with each problem, it is proposed to use a visible filter for color correction, an antireflection film, an infrared ray-shielding filter, and an electromagnetic wave-shielding filter, etc., on the front of displays.
In particular, color correction by a filter is described in Japanese Patent Laid-Open Nos. 153904/1983, 188501/1986, 231988/1991, 205643/1993, 145918/1997, 306366/1997 and 26704/1998 and WO98/23980.
Various kinds of producing methods of optical filters are disclosed. In the optical filters disclosed in Japanese Patent Laid-Open Nos. 188501/1986, 205643/1993, 145918/1997 and 306366/1997, dyes or pigments are added to transparent supports to make the supports function as filters. In the optical filter disclosed in Japanese Patent Laid-Open No. 26704/1998, the hard coat layer (the surface-hardening layer) provided between the transparent support and the antireflection layer is colored to make the hard coat layer function as a filter. In the optical filter disclosed in Japanese Patent Laid-Open No. 209510/1995, a mixed solution of a dye and a metal alkoxide are coated, dried and burned to make it function as the filter layer. Further, in the optical filter disclosed in WO98/23980, a dye and a matrix polymer is dissolved in an appropriate solvent and coated to make it function as the filter layer.
However, the filter layers produced by these methods come to reflect light of specified wavelength when the concentration of the dyes in the filter layers becomes high, which deteriorates color correcting performance of the optical filter. Contrary to this, when the dye concentration in a filter layer is lowered, the thickness of the filter layer should be increased to obtain the optical concentration as the optical filter. Accordingly, in the production process of filter layers by coating, troubles such as drying failure and cracking of films are liable to occur.
Further, multilayer antireflection films produced by laminating transparent thin films of metal oxides, which are used for spectacle lenses, camera lenses, etc., are used for preventing reflection, and the more the layer number, the more effective is the multilayer antireflection film to the lights of a wide wavelength range. Therefore, multilayer lamination of transparent thin films by a vacuum deposition method, a sputtering method, an ion-plating method, a CVD method, or a PVD method has been performed.
However, since a high degree of vacuum is necessary in these methods, equipment becomes large-scaled, further it is required to exactly control the thickness of each layer according to the relationship between the refractive indices and the film thicknesses previously designed, therefore, there is a problem in the productivity and it is difficult to produce an antireflection film on a large size film inexpensively.
Further, since films are deformed and denatured by heat, they cannot be exposed to a high temperature, in addition the volatile components (water, solvents, etc.) of films hinder maintaining a high degree of vacuum, so there arises a problem that the film thickness and strength are liable to become uneven when transparent thin films each having a different refractive index are laminated on a film by a vacuum deposition method or a sputtering method.
For solving such problems due to a vacuum deposition or sputtering method, a method of forming an antireflection film by wet coating has been discussed. For example, an antireflection transparent material containing microvoids and an inorganic fine particles of a diameter of from 1 to 300 nm and having an antireflection layer having a refractive index lower than that of the substrate is disclosed in Japanese Patent Laid-Open No. 59250/1985. However, even with to this technique, the obtained sample must be subjected to treatment with an activated gas under vacuum to lower the refractive index, which encounters the same problem as the deposition (high vacuum system and delicate control of activated gas treatment).
Further, in Japanese Patent Laid-Open Nos. 49501/1984, 50401/1984 and 48527/1995, antireflection films having a low refractive index layer using silica particles and a silane hydrolyzed product as the binder are disclosed. However, in this constitution, it is difficult to suppress the refractive index of the low refractive index layer sufficiently low, thus such a problem as a low refractive index cannot be obtained still remains.
A variety of subjects are required of these filters respectively by the kinds of displays, e.g., a visible filter for color correction is required to form a sharp absorber corresponding to the characteristics of the display elements, in addition, strengthening of a heat resisting property by glass incorporation, etc., and physical strengthening are required. Further, it is necessary for an antireflection film to have multilayer structure for achieving ideal refractive indices in every range of visible light, however, forming a multilayer film by deposition or coating is accompanied by processing difficulties and an economical problem. Accordingly, when various functions are tried to be given to the front panel of a display, there arises the restriction that one function must not hinder other functions in addition to the fact that filters having the respective functions are required to have respective characteristics. Therefore, a multifunctional front panel has not been put to practical use as yet.
Further, the front panel having an antireflection layer and a selectively absorptive filter layer is required to have strength of more than a certain degree since not only the front panel must prevent the scattering of display elements at the back of the front panel at the time of breakage but also it has to be sufficiently resistive to the breakage of its own.
Accordingly, an object of the present invention is to provide a multifunctional optical filter which satisfies the requirements such as excellence in production aptitude, non-bulk, light weight and high mechanical strength, and a front panel and an image display using the same, in particular, to provide an optical filter having a proper color correcting performance in addition to the antireflection performance, and a front panel and an image display using the same.
Another object of the present invention is to provide an optical filter capable of selectively cutting the light of the wavelengths which reduce color purity to correct color balance, and to provide an image display using the same.
A further object of the present invention is to provide an optical filter which can suppress the reduction of visibility due to the reflection of external light on a display panel, can regulate colors and is improved in resistance to breakage.